Hello Kim,
Are you trying to decide which new firewall model to buy? Unfortunately there is not a straight answer or a simple formula for you to use here.
You can just look at data sheets and bandwidth for any vendor but that may not lead you to the best conclusion if you're not fully informed of all the variables, like features that you want to use, impacts of the features on firewall throughput, nature of your traffic, trends etc. Internet bandwidth is just one of several parameters to consider.
The best is to have a discussion with a cisco pre-sales, local partner or reseller, to review your requirements and plans first. Pre-sales engineers do this sort of thing all the time and can dive into your requirements in more depth, starting from evaluation of how much traffic your current firewall is handling.
thank you and have a good day!
